In recent times, it has been known to use conductive fiber mats in automobile plastic parts for the purposes of strengthening the plastic and to shield the car radio from electromagnetic engine emissions.
In the field of electronic packaging, electrostatic free bags have been constructed using carbon-impregnated plastic materials and metal foil.
Perforated metal foil has provided electronic envelopes with a certain degree of optical transparency, i.e., usually about 40 percent transparency.
When aluminum screens are used, the obtained optical transparency may be as much as 60 percent, but the laminate is generally inflexible.
This invention contemplates the use of a carbon-fiber mat as a middle layer disposed between two outer layers of electrostatic free, low-density polyethylene material to form an optically transparent and flexible laminate for electronic envelopes or packages requiring EMI and RFI shielding. The mat of electrically conductive fiber has a low density of about 0.2 ounces per square yard to about 0.5 ounces per square yard. Utilizing this low density material, optical transparencies approaching 70 to 80 percent can be achieved, particularly if the fibers are metal coated.
Excellent electrical shielding properties are achieved with fibers having a high length to diameter ratio, such as 0.75 inches or greater in length, to 7 or 8 microns in diameter.
The fibrous mat has a typical resistivity of less than one ohm centimeter.